The technology of optical data storage media embraces a broad range of materials and signal mechanisms, including media where recording takes place before media production ("read-only"), media on which data can be directly recorded and becomes permanently fixed ("write-once"), and media which can be both recorded upon and erased ("erasable"). Optical signals fall within three general categories: reflective, transmissive, and absorptive. These signals may be produced in a variety of ways, including the use of pits (or bumps) or holes in certain layers of the media, optical density change materials (such as photographic films, photoresists, and photopolymers, which undergo optical density changes upon absorption of light), phase change materials (undergoing a transition from a crystalline to an amorphous state or vice versa upon absorption of light), magneto-optical materials (where signals are recorded by localized heating under a magnetic field to change the direction of magnetization), and ablative thin films (where the recorded pattern induces light amplitude modulation).
Many of these techniques are examples of thermooptical recording, in which light from a laser is focused on a small, usually diffraction limited spot at a specified depth in the medium. The energy from the focused light heats the spot and effects the change which functions as data storage.
The construction of the medium will vary depending on the type of signal to be recorded on it or 5 built into its structure during fabrication. Optical media in general have a multilayer construction. 0f these, some are constructed with contoured (e.g., grooved or pitted) interfaces between the layers either as the source of the signals or as a means for guiding the reading or recording beam to keep it on track. Others are constructed with layers of differing absorptivity.
Examples of optical data storage media which include layers of differing absorptivity are those described in European patent application Publication No. 136070, published on Apr. 3, 1985, entitled "Erasable Optical Data Storage Medium and Method and Apparatus for Recording Data on the Medium" (Optical Data, Inc.); and U.S. patent application Ser. No. 07/153,288, filed on even date herewith. inventors B. Clark, J. Finegan and R. Guerra with the same assignee named herein, entitled "Optical Data Storage Media for Substrate Incident Recording." In such media, binar optical data appear as pits or bumps in an otherwise flat reflecting surface. which may either be a partially reflecting interface between two layers of different refractive indices, or a fully reflecting surface such as a metallic film. Reading of the data is accomplished by passing a laser beam over the pits or bumps and monitoring the intensity of the reflected light. Each pit or bump varies the optical path of the beam, thereby lowering the reflected intensity either by destructive interference when combined with a second beam (not reflected off a pit or bump) or by scattering due to the curvature of the pit or bump.
The formation of these pits or bumps during data recordation is attributable to the structure of the medium, which combines an expansion layer with a retention layer. The expansion layer absorbs energy from a high intensity record beam and expands with the resulting rise in temperature to bulge out to one side. The retention layer converts from a glassy state to a rubbery state upon heating, then back to a glassy state upon cooling. The two layers are configured in such a manner that the retention layer heats up with the expansion layer, becomes rubbery, and conforms to the bulge, then cools back to its glassy state before the expansion layer can cool sufficiently to cause the bulge to retract. The bulge is thus fixed by the retention layer, and serves as the pit or bump which constitutes the data. Erasure of the data is achieved by an erase beam which is absorbed only by the retention layer. The absorbed beam heats the retention layer to its rubbery state, permitting elastic forces in the expansion layer and viscoelastic forces in the retention layer to return both layers to flatness.
One of the difficulties in recording data on these and other thermo-optically recordable media is that the diffraction-limited optics within the recorder limit the beam to a finite width within the media which in some cases may be larger than the desired recording mark. This is especially true of multilayered media such as those described above in which the light absorptivity in adjacent layers differs, and in which the energy deposition in the more absorptive layer occurs through the depth of the layer. This is true both where the thermo-optical layer is the layer of greater absorptivity, and where the thermo-optical layer is the layer of lesser (or zero) absorptivity or some other layer in the medium. In the latter case, the more absorptive layer acts as a filter.
Further difficulties are encountered in multilayered media which use a grooved interface as a means for tracking. In such media, tracking is achieved by monitoring the intensity of light reflected off the interface. The groove depth is such that light reflecting off the top of the groove and light reflecting off the bottom of the groove combine to give a total reflectance, and either a broad beam designed to overlap the sides of the groove or a combination of beams on and off the groove are used, such that the overall reflected intensity varies depending on how well the beam(s) are centered on the track. In media where the index of refraction on both sides of the interface is very close, reflection is weak and the sensitivity of the tracking mechanism is accordingly limited.
The present invention is an improvement over all such types of media. This invention combines interface height variations (i.e., variations in the relative thicknesses of the two layers on either side of the interface) with a differential in light absorptivity between the two layers. The advantages flowing from this combination are numerous, depending on the type of medium to which the invention is applied as well as the type and arrangement of the height variations themselves.
The height variations may for example assume the form of grooves or ridges superimposed over the tracks. In media with layers of different absorptivities, where the more absorptive layer is thermo-optically active, the height variations may form ridges in the more absorptive layer in the track regions, thickening the layer to produce greater light absorption and therefore greater energy deposition in these regions. This increases the degree to which the signal energy is focused in the track. In media where the more absorptive layer is a filter rather than thermo-optically active, a similar result can be achieved, the height variations will be formed into grooves rather than ridges so that the regions over the tracks are thinner, permitting more energy to pass through them to the thermo-optically active layer again imparting added spatial control. In media where more than one thermo-optically active layer are present, ridges or grooves provide a means for controlling where energy in each of the layers is absorbed. In media where grooves are already present for tracking but without differential absorptivities, the addition of differential absorptivity heightens the reflected intensity differential between the on-track and off-track areas, increasing the tracking sensitivity.
Alternatively, the height variations may assume the form of a series of discrete protrusions or depressions along a portion of the track. These protrusions or depressions will be arranged in distinctive sequences to provide the medium with a format for use during recordation and as a guide or template for producing an analogous data sequence in the medium while other data is being recorded on the medium. The data sequences produced from these protrusions or depressions may serve as a format during playback of the recorded data in much the same way as format sequences on standard compact disks.
Still further, both types of height variations may be included in a single medium.
Further advantages and embodiments of the invention will be apparent from the following description.